Krishna V. Subbarao

Krishna V. Subbarao was born in 1958 in India. He obtained his B.S. and M.S. degrees in plant pathology from the University of Mysore, India, and a Ph.D. degree in plant pathology from Louisiana State University (LSU). After short post-doctoral stints at LSU and the University of California-Berkeley, Subbarao joined the faculty of the Department of Plant Pathology at the University of California-Davis, where he is currently a full professor. His research focuses on the etiology, epidemiology, and impact of altered production practices on pathogen ecology and on elucidating mechanisms of disease suppression attained via cultural manipulations. He previously served as associate editor and senior editor of Phytopathology and has served either as a member or chair of a number of APS committees.

Subbarao’s impressive career achievements have come through the study of two fungal pathogens, Sclerotinia spp. and Verticillium spp. He has utilized these systems to develop pioneering research programs in the assessment of changing production practices on plant diseases. Subbarao early in his career demonstrated that the benefits of deep-plowing to bury pathogen propagules were not commensurate with the cost, and the meager benefits were limited to a single production season. This is one of the best-documented examples linking effects of farming practices with the distribution of pathogen propagules and is notable for the fact that it is not merely important to develop effective disease management strategies, but also to abandon those that are inefficient. In a series of articles, he developed a highly innovative, yet very practical knowledge base of soilborne disease management protocols. These include, among others, the effect of combining subsurface irrigation and minimum tillage on inoculum and disease dynamics, the mechanisms by which soil suppressiveness is achieved and whether this method is more economical than fungicide applications. The framework developed has become the standard approach for managing diseases caused by the Sclerotinia species.

A second area of Subbarao’s widely acclaimed contributions has come from his studies concerning the ecology of Sclerotinia. His research provided answers to long-standing questions concerning the relative importance of two Sclerotinia species in California. Subbarao’s group demonstrated how differential survival in soil, environmental requirements for spore production, and cultural practices help explain the prevalence of S. sclerotiorum in the San Joaquin Valley and its rarity in the Salinas Valley, while the reverse pattern holds for S. minor. The fastidious soil moisture conditions required for apothecial production in the Salinas Valley are met during the rainy season, but the main host, lettuce, is absent (due to the mandated “lettuce-free” period to break the Lettuce mosaic virus vector life cycle) or when present, is at growth stages unsuitable for infection. This is one of the rare examples of legislative action to control one disease having the inadvertent beneficial effect of suppressing another threatening disease. This meticulous work over the past decade has provided a lucid understanding of the reasons for the unique niches occupied by Sclerotinia species. His group has also characterized the process by which ascospore maturity of S. sclerotiorum is synchronized, providing new insights into the reproductive biology of this pathogen.

A third area of significant and ongoing accomplishment is the study of Verticillium wilt caused by Verticillium dahliae. This pathogen has great genetic plasticity, can infect more than 200 plant species, and the list of potential hosts is continually expanding. One example of host range expansion occurred in lettuce where entire crops were lost to Verticillium wilt. Subbarao’s research on V. dahliae has offered insights into both pathogen biology and sound information germane to disease management. The former includes a particularly elegant study, showing differential development of the pathogen in susceptible and resistant lettuce cultivars. Using a strain that had been transformed to express a fluorescent protein, Subbarao compared the colonization processes between susceptible and resistant lettuce cultivars. This work showed that whereas the resistant cultivar sustained feeder root infections, the host prevented pathogen entry into the tap root and the stem. These findings extend our understanding of how V. dahliae interacts with susceptible and resistant hosts, while also having direct relevance to a disease problem affecting a crop of significant economic importance. Subbarao was also instrumental in a multi-institutional effort to sequence the genomes of V. dahliae and two other species of Verticillium that were released recently.

Finally, Subbarao’s contributions have also come from his stellar work on developing rotation strategies for pathogens possessing broad host ranges. In a series of well-conceived studies, he determined that broccoli cultivation not only reduced resident soil propagules of fungi, but also wilt incidence and severity in subsequent susceptible crops. Unlike soil fumigation, rotations with broccoli did not eradicate the pathogen, but maintained soil propagules below the threshold at which crop losses accrue, despite the cultivation of susceptible crop plants. He then obtained empirical data on the mechanisms of broccoli-induced V. dahliae suppression using a combination of soil microbiology, soil chemistry, and molecular cytology. He demonstrated that not all glucosinolates and their catabolic products are involved in pathogen suppression, and that V. dahliae suppression by broccoli involves multiple factors (e.g., the type of glucosinolates and their catabolic products, soil microbial communities). This interdisciplinary work lays the foundation for economically effective disease management in this pathosystem.

While this brief synopsis of Subbarao’s career achievements provides a sense of the vitality and versatility of his research, it is by no means comprehensive. Subbarao possesses a prolific publication record with more than 120 refereed journal articles, reviews in Annual Review of Phytopathology (2009), and other influential journals, invited book chapters, and numerous articles in trade journals. He has also overseen the research of staff, undergraduates, graduate students working toward their Ph.D. degrees, and 15 post-docs from diverse cultural backgrounds. His former associates are pathologists in government and academia, with four being faculty in land-grant universities. He is quiet but intensely deliberative and is imbued with the finest human qualities. His outstanding achievements in his distinguished career eminently qualify him for the APS Fellow Award.